tests: run most tests via tests/Makefile.am

tests: run most tests via tests/Makefile.am

* Makefile.am (SUBDIRS): List tests after ".".
Move most test-related things from here to ...
* tests/Makefile.am: ... here.
* configure.ac (AC_CONFIG_FILES): Add tests/Makefile.
* tests/helin-segv: Adjust.
* tests/help-version: Likewise.
* tests/hufts: Likewise.
* tests/memcpy-abuse: Likewise.
* tests/mixed: Likewise.
* tests/null-suffix-clobber: Likewise.
* tests/stdin: Likewise.
* tests/trailing-nul: Likewise.
* tests/zdiff: Likewise.
* tests/zgrep-f: Likewise.
* tests/zgrep-signal: Likewise.
* tests/znew-k: Likewise.